:screenshot <filename.png> --fullpage saves a screenshot to your downloads directory using the optional file name. If no filename is included, the file name will be in this format:

Screen Shot date at time.png

The --fullpage parameter is optional. If you include it, the screenshot will be of the whole page, not just the section visible in the browser windows. The file name will have -fullpage appended to it as well. See Web Console Helpers for all parameters.